Field of the Invention
This invention relates to a method of producing a silicon compound powder useful as the material of engineering ceramics, and more particularly to a method of continuously producing a powder of silicon carbide or silicon nitride by reduction of silica.
Recently a strong interest is shown in so-called engineering ceramics as substitutes for conventional heat-resistant metal materials with a view to saving energy and resources of rare metals. Among hitherto tested materials for such ceramics, carbon silicide or silicon carbide and trisilicon tetranitride are deemed especially promising and have been used in some engineering ceramics products already developed to the practical application stage.
The following reactions are representative of known methods for preparing silicon carbide or silicon nitride. ##STR1##
Silicon-containing nonoxide ceramics materials such as SiC and Si.sub.3 N.sub.4 are not easy to sinter. Therefore, these materials must be prepared in very fine powder form with a mean particle size of about 1 micron or smaller. Besides, very high purity is required of these materials in view of the high temperature strength of the sintered ceramics. In general, a purity of 99.5% or above is required. From these points of view, the above classified production methods are evaluated as follows.
The method (A) is rather unfavorable for industrial practice because it is difficult to obtain the product in desirably fine powder form, and also because high purity silicon is a costly material. The vapor phase reaction method (B) provides very fine powders of high purity silicon carbide or silicon nitride, but the silicon compounds used in these reactions are costly and the yields of the intended compounds are low. Besides, in general it is difficult to produce crystalline silicon carbide or silicon nitride by these vapor phase reactions. The thermal decomposition method (C) provides high purity products, but the starting silicon compounds are very costly.
The reduction method (D) is favorable for industrial practice because of using inexpensive silica as the primary material. However, this method cannot be deemed fully developed because the obtained powders are relatively coarse and also because the productivity is relatively low.
The silica reduction reactions of equations (7) and (8) are usually carried out batchwise. As a continuous process, it was proposed to carry out a moving bed type reaction in a vertical furnace using graphite as heating elements. However, this method involves various problems most of which are not easy to solve. First, the productivity does not become so high as is expected because the raw material needs to be subjected to reaction in the form of large granules or pellets (at least about 10 mm in diameter) to realize a moving bed and, therefore, must stay in the furnace for a considerably long time until completion of the reaction in the core region of each pellet. Second, it is inevitable that the distribution of temperature in the vertical furnace becomes nonuniform in the horizontal directions. This is unfavorable to the uniformity of the partial size of the product. The distribution of temperature may be made uniform by decreasing the diameter of the reaction tube, but this is contrary to the intention of developing an industrially practicable and efficient technique. Besides, various means and subsidiary apparatus must be devised to keep up continuous operation for a long period without suffering from deposition or fusion-adhesion of silicic scale onto the reaction apparatus. Also it is troublesome to finely pulverize the product obtained in the form of a coarse powder.
A more serious problem in the continuous production of silicon carbide or silicon nitride powder in a vertical furnance is a blocking or clogging phenomenon. In the above described method a column of the granulated or pelleted material moves downward in the furnace core tube made of graphite. As the reaction proceeds, some portions of the granules or pellets collapse and powder by the action of the deadweight thereof. As a result, an obstruction is offered to smooth downward movement of the reacting material and/or to uniform flow of the gas in the tube. In an extreme case, clogging of the tube with the collapsed material causes the material to blow up so that the operation can no longer be continued. Also it is a matter of disadvantage that silica in the raw material reacts with the graphite core tube too to result in early wear of the core tube.
Japanese patent application provisional publication No. 54-134100 (1979) and Japanese patent application publication No. 58-48487 (1983) propose to produce a powder of .beta.-SiC by confining a powder mixture of silica and carbon in a cylindrical container made of high density graphite and externally heating the container while it is moved horizontally in a tubular furnace. This method can be practiced as a continuous process by using a plurality of containers without suffering from clogging of the reaction tube or the furnace. However, in this method it is impossible to discharge carbon monoxide gas formed by the reaction from the reaction system since the reaction system is confined in the closed container. The accumulation of carbon monoxide gas in the reaction system causes the rate of reaction to be gradually and significantly lowered. For this reason this method is not suited to industrial practice.